The present invention relates generally to the field of syntheses of alpha-beta unsaturated acids, such as methacrylic acid, from saturated acids, such as isobutyric acid, and more particularly to syntheses of such materials using molybdophosphoric acid catalysts.
Methacrylic acid and methacrolein are chemical species which are fundamental to the plastics industry. Methacrylic species such as methacrylic esters, are used in huge quantities worldwide for diverse employment in the elaboration and formulation of structural, coating, aesthetic and other polymerizable resin systems. Accordingly, the efficient synthesis of methacrylic acid and methacrylic precursors such as methacrolein is greatly desired.
Isobutyric acid is a readily available material having the requisite carbon atom structure for transformation into methacrylic acid. Such transformation, however, requires both oxidation and dehydration. Accordingly, catalytic agents which accomplish this transformation in a selective and efficient manner are highly desired.
Heteropolyacids are a recognized class of acids containing large amounts of oxygen and hydrogen, and multiple atoms of one or more elements, such as molybdenum or tungsten, surrounding one or more heteroatoms of another element, such as phosphorous. Polyanions of such acids consist primarily of octahedral MoO.sub.6 or WO.sub.6 groups, so that the conversion of [MoO.sub.4 ].sup.2- or [WO.sub.4 ].sup.2- into polyanions requires an increase in coordination number. Cotton and Wilkinson "Advanced Inorganic Chemistry", 4th edition, pp. 852-861 Wiley & Sons, N.Y. (1980), disclose that heteropolyanions can be formed either by acidification of solutions containing the requisite simple anions, or by introduction of the hetero element after first acidifying the molybdate or tungstate. As indicated at Table 22-C-2 of Cotton and Wilkinson (pg. 857), various heteropolyanion formula types are known.
Heteropolyacids such as molybdophosphoric acids, are known to exist in the stoichiometry of the "Keggin" structure (PMo.sub.12 O.sub.40.sup.3-), as well as in the stoichiometry of a Dawson structure (P.sub.2 Mo.sub.18 O.sub.62.sup.6-). Of these structures, the "Keggin" structure is the most commonly formed cluster, and Keggin structure molybdophosphoric acids are known to be suitable vapor phase catalysts in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. In Japanese Patent Disclosure Number 1975-4014 dated Jan. 16, 1975 entitled "A Process for Manufacturing Methacrylic Acid, abstracted at Chemical Abstracts, Volume 83, 4408b (1975), the use of molybdophosphoric acid having the empirical formula H.sub.3 Mo.sub.12 PO.sub.40.nH.sub.2 O, as well as molybdovanadophosphoric acid, are disclosed for use in vapor phase oxidative dehydrogenations of isobutyric acid. Such reactions are performed in the presence of oxygen and other gases such as nitrogen, steam, etc., such reactions being carried out in the temperature range of 200.degree.-400.degree. C., preferably 250.degree.-350.degree. C. More particularly, this Japanese patent disclosure indicates that the selectivity of methacrylic acid can be improved by using a catalyst which is prepared by adding a sulfate of an alkali metal, nickel or cobalt to a heteropolyacid.
It has long been known to use various heteropolyacids to catalyze certain organic reactions. For example, in U.S. Pat. No. 4,192,951, vapor phase oxidation procedures are disclosed utilizing various heteropolyacid catalysts, including heteropolymolybdic catalysts containing vanadium, tungsten, tantalum or niobium. Such compounds act as catalysts for the synthesis of materials such as maleic acid and acetic acid. U.S. Pat. No. 4,192,951, also discloses a molybdophosphoric acid catalyst having an empirical formula of H.sub.6 [P.sub.2 Mo.sub.18 O.sub.62 ] which was prepared using a procedure involving the refluxing of Mo.sub.3 and H.sub.3 PO.sub.4 overnight to produce a bright yellow filtrate. Although the empirical formula provided relating to the molybdophosphoric acid catalyst of the '951 disclosure corresponds to the empirical formula of a Dawson structure catalyst, no mention is made in the '951 patent of the stoichiometry of the structure obtained in Example 1. It is clear from the filtrate color reported in the '951 patent that the stoichiometry of the '951 catalyst is not of the "Dawson" type. In an article entitled "Contribution To The Chemistry of Phosphomolybdic Acids, Phosphotungstic Acids, and Allied Substances", by Hsein Wu, J. Biol. Chem., 43, 189 (1920) a proper procedure for preparing phospho-18-molybdic acid of the Dawson structure is disclosed. As explained by Wu at pages 196 and 197, care must be taken during the preparation of such an acid to avoid the formation of yellow crystals and to obtain orange crystals which are indicative of phospho-18-molybdic acid of the Dawson structure.
In U.S. Pat. No. 4,146,574 entitled "Process For Preparing Heteropolyacids", various heteropoly-acids suitable as components in certain oxidation reactions are disclosed. Such catalysts are described as facilitating the oxidative dehydrogenation of isobutyric acid to methacrylic acid, the oxidative dehydrogenation of methyl isobutyrate to methyl methacrylate and methacrylic acid, the oxidative dehydrogenation of isobutyraldehyde to methacrolein and methacrylic acid, the oxidation of methacrolein to methacrylic acid, and the oxidative dehydrogenation of methylisopropyl ketone to methylisopropenyl ketone. Typically, catalytic reactions of the type disclosed in U.S. Pat. No. 4,146,574 are conducted using a mixture of gases, such as steam, oxygen, and nitrogen which are permitted to contact a catalytic substrate for preselected contact times at preselected reaction temperatures.
While the above-described methods for converting isobutyric acid to methacrylic acid have achieved some success, a need still exists for methods for efficiently and selectively converting isobutyric acid to methacrylic acid.